In pneumatic vehicle tires such as those for passenger cars, trucks, and buses, a belt layer composed of a steel cord is arranged on a carcass layer to receive the strong shock and load subjected during running of a tire. One problem to be solved in a pneumatic vehicle tire is to improve the processability of a rubber composition for covering a steel cord to improve the productivity of a tire. From this viewpoint, it has been tried to increase the amount of a process oil to be compounded or to decrease the amount of a carbon black to be compounded to decrease the viscosity of unvulcanized rubber composition. However, these proposals have the problems of decreasing the properties, such as hardness, of the vulcanized rubber. Furthermore, it may be possible to add processing aids, processing aids capable of providing the sufficient results have not been known yet.
Further, it has been tried to increase the compounding amount of a process oil and to decrease the compounding amount of a carbon black for the purpose of improving the processability and productivity of rubber compositions for carcass coat or belt cover or bead filler or further for inner liner of a pneumatic tire such as for passenger cars, trucks and buses, so as to decrease the viscosity of the unvulcanizer rubber composition. However, these processes cause the problems of decreasing the physical properties of vulcanizates such as hardness, E' (storage modulus), tan.delta., although the viscosity of the unvulcanized rubber composition is decreased. Furthermore, it is also considered to use the addition of a processing aid, there are no existent processing aids, in the prior art, capable of attaining the above purpose without impairing the vulcanized physical properties.
Furthermore, as one problem to be solved in a tire for a passenger car, truck, bus etc. is to decrease the rolling resistance of a tire and also to decrease the heat generation of a tire. From this viewpoint, it has been proposed that, as a reinforcing agent for a rubber composition for a side tread of a tire, a special carbon black is used or a part of the carbon black is replaced with silica. However, when the silica is used as a reinforcing agent, there occur problems such as the mixing processability of unvulcanized rubber compounding composition and the dispersibility thereof becomes poor with the increase in the compounding amount of silica, although the rolling resistance can be decreased.
Furthermore, a rubber compounding composition containing clay (i.e., water-containing aluminum silicate) is used as a rubber for, for example, a head insulation and a white side tread of a tire such as a passenger car, truck, bus. Such a rubber composition is desired to improve the processability (e.g., the prevention of burning of rubber at, for example, extrusion step and compression step, the improvement in the production line speed) and to increase the productivity. For this purpose, although it is considered that the compounding amount of process oil is increased, or the compounding amount of carbon black is decreased, there occur problems such as the vulcanized physical properties such as hardness and strength are impaired, although the viscosity of the unvulcanized compounding composition is decreased. Furthermore, processing aids capable of attaining the above purpose is not known in the art.